The present invention relates, in general, to variable gain amplifiers and, in particular, to an amplifier in which the gain is varied by varying the input impedance.
Variable gain is required in a radio frequency amplifier. This variable gain usually is made up of a combination of continuous gain (i.e., analog variable) or discrete gain (i.e., digitally variable).
Continuously variable gain, as in an analog variable gain circuit, is typically used to achieve instantaneous dynamic range or to compensate for temperature variations. Discretely variable gain, on the other hand, is typically used to compensate for process variations by adjusting the gain of the radio frequency transmitter at the factory. The capability to adjust the gain of the radio frequency transmitter in discrete steps is used to compensate for not only the process variations in the transmit chip but also any off chip components, such as filters and power amplifiers.
Conventional methods of achieving discrete gain variation involve using a switch and resistor arrangement as in a step attenuator. The drawbacks of a conventional step attenuator are noise and power dissipation.
It is an objective of the present invention to provide a new an improved variable gain amplifier.
It is another objective of the present invention to provide a variable gain amplifier having an improved dynamic range.
It is a further objective of the present invention to provide a variable gain amplifier in which linearity is maintained over the dynamic range.
It is yet another objective of the present invention to provide a variable gain amplifier in which the output match remains constant over the dynamic range to maintain monotonicity, namely the gain is either continuously increasing or decreasing versus a continuously increasing or decreasing control.
An amplifier having a variable gain and a fixed input impedance, constructed in accordance with the present invention, includes a transistor having a base, a collector and an emitter and an inductor coupled to the emitter of the transistor. This amplifier also has a first plurality of reactance/switch pairs connected in parallel with the inductor and a second plurality of reactance/switch pairs connected in parallel between the collector and the base of the transistor. An amplifier having a variable gain and a fixed input impedance, constructed in accordance with the present invention, further includes means for closing a selected switch of the first plurality of reactance/switch pairs to change the gain of the amplifier and a selected switch of the second plurality of reactance/switch pairs to maintain the input impedance of the amplifier fixed.
A differential amplifier having a variable gain and a fixed input impedance, constructed in accordance with the present invention, includes a first transistor having a base, a collector and an emitter, a second transistor having a base, a collector and an emitter; a first inductor coupled to the emitter of the first transistor; and a second inductor coupled to the emitter of the second transistor This differential amplifier also includes a first plurality of reactance/switch pairs connected in series with the first inductor with the series connections of the first inductor and the first plurality of reactance/switch pairs connected in parallel with the second inductor and with the second inductor with the series connections of the second inductor and the first plurality of reactance/switch pairs connected in parallel with the first inductor. A differential amplifier having a variable gain and a fixed input impedance, constructed in accordance with the present invention, further includes a second plurality of reactance/switch pairs connected in parallel between the collector and the base of the first transistor and a third plurality of reactance/switch pairs connected in parallel between the collector and the base of the second transistor. This differential amplifier also includes means for closing a selected switch of the first plurality of reactance/switch pairs to change the gain of the differential amplifier and a selected switch of the second plurality of reactance/switch pairs and a selected switch of the third plurality of reactance/switch pairs to maintain the input impedance of the differential amplifier fixed.